The base of the upper Miocene Puente Formation marks the largest velocity contrast in the Los Angeles (LA) basin. In the western and central basin, this is the contact between the Nodular Shale and schist basement, coarse clastic strata of the Topanga Formation, mafic volcanics, or "Schist'Conglomerate." In the eastern and southeastern basin, the top of the Topanga is commonly marked by the El Modeno Volcanics; the presence of volcanics requires a velocity contrast between Topanga and Puente even where volcanics are not present. Elsewhere, the pre-upper Miocene rocks are Mesozoic granite and pre-Miocene sedimentary rocks.
Our map covers the basin west of a line connecting Las Cienegas, Dominguez, and onshore Wilmington oil fields (Figure 1). West of the Newport-Inglewood fault and south of the northern LA fold-thrust belt, this is the top of Catalina Schist basement. Elsewhere, it is the top of Topanga Formation, which consists of indurated clastic strata and basaltic volcanic rocks. The Topanga is of unknown thickness within the fold-thrust belt and beneath the LA trough; we chose to present data on what is known rather than what is speculated. The map shows wide variability along strike. For example, Tom Wright's Brentwood cross section, used to infer a lens focusing of Northridge earthquake waves to produce greater damage along I-10 in northern LA, is valid only for Brentwood and does not apply to Santa Monica or Sawtelle-Cheviot Hills. Vertical relief between the LA trough is great at Inglewood but much less at Potrero and Rosecrans to the southeast. These variations show the complexity of the surface and the importance of modeling ground motion using 3D geometry for wave propagation.
We first contoured the surface using well and surface control. The contoured data were then used to create an XYZ dataset by triangulation (TIN), which was then iteratively densified and adjusted to honor the original data and geological interpretation. The densified TIN was then gridded with a continuous-curvature splines-in-tension algorithm. We also made a video "fly-through" of the gridded basement surface. The data, documentation, control point maps, and movie will be available on our website at <http://pandora.oce.orst.edu>
